One of the stereo recording systems is a mid-side (MS) stereo recording system. The MS stereo recording system includes two microphones, i.e., a mid microphone (hereinafter, its output signal is referred to as “signal M”) and a side microphone (hereinafter, its output signal is referred to as “signal S”). The mid microphone has unidirectionality or narrow directionality, and is disposed so as to face the center of a sound source. The side microphone has a bidirectionality, and is disposed perpendicularly to the direction of the sound source. The sum M+S of and the difference M−S between the signals M and S are calculated, and converted to stereo signals for right and left channels, respectively, and the stereo signals are then output. The mid and side microphone units are accommodated in one microphone casing into an MS stereo microphone.
FIG. 12 illustrates a typical conventional MS stereo microphone. In FIG. 12, a mid microphone unit 20 and a side microphone unit 30 are capacitor microphone units. The mid microphone unit 20 includes a diaphragm 201 and a fixed electrode 202 opposed to each other at an appropriate space so as to define a capacitor. The side microphone unit 30 also includes a diaphragm 301 and a fixed electrode 302 defining a capacitor. The mid microphone unit 20 is disposed to face the front in a microphone casing (not shown). The side microphone unit 30 is disposed sideways in the microphone casing, namely, disposed perpendicularly to a sound collection axis of the mid microphone unit 20. The mid microphone unit 20 has unidirectionality or narrow directionality. The side microphone unit 30 has bidirectionality.
The diaphragms 201 and 301 of the mid and side microphone units 20 and 30, respectively, are connected to ground GND. In the mid and side microphone units 20 and 30, the diaphragms 201 and 301 vibrate upon receiving sound waves, and their distances from the fixed electrodes 202 and 302 vary, leading to variations in capacitance. Such variations in capacitance are output from the fixed electrodes 202 and 302 as sound signals. The sound signals output from the fixed electrode 202 of the mid microphone unit 20 are converted to low-impedance signals by an impedance conversion circuit mainly including an FET 40. The impedance conversion circuit further includes a bias circuit 10 and a load resistance 41. A DC supply voltage VDD is applied to the drain of the FET 40.
In this circuit configuration, the sound signals, which are output from the mid microphone unit 20 and subjected to impedance conversion, are sent to a buffer amplifier mainly including a transistor 45 through a coupling capacitor 42. Signals output from the buffer amplifier are sent to a positive terminal of each of primary windings of matrix transformers 70 and 80 through a coupling capacitor 48. The DC supply voltage VDD is divided by resistive dividers 43 and 44, and such a divided voltage is applied to the base of the transistor 45. The DC supply voltage VDD is applied to the collector of the transistor 45 through a resistance 46. The emitter of the transistor 45 is connected to the ground GND through a resistance 47. A positive terminal of a secondary winging of the matrix transformer 70 is connected to an output terminal L of a left channel. A negative terminal of the secondary winging is connected to the ground GND.
A circuit for the side microphone unit 30 also includes an impedance conversion circuit mainly including an FET 60 and a buffer amplifier mainly including a transistor 65, as in the circuit for the mid microphone unit 20. The collector of the transistor 65 defining the buffer amplifier outputs negative-phase signals −S from the side microphone unit 30. The emitter of the transistor 65 outputs positive-phase signals +S from the side microphone unit 30. The circuit further includes a bias circuit 50 of the impedance conversion circuit, a load resistance 61, a coupling capacitor 62 between the impedance conversion circuit and the buffer amplifier, resistive dividers 63 and 64, a resistance 66 connected between a DC power supply and the collector of the transistor 65, and a resistance 67 connected between the emitter of the transistor 65 and the ground GND. The negative-phase signals −S from the microphone unit 30 are output from the collector of the transistor 65, and then sent to a negative terminal of the primary winding of the matrix transformer 70 through a coupling capacitor 68. The positive-phase signals +S from the microphone unit 30 are output from the emitter of the transistor 65, and then sent to a negative terminal of the primary winding of the matrix transformer 80 through a coupling capacitor 69. A positive terminal of a secondary winging of the matrix transformer 80 is connected to an output terminal R of a right channel. A negative terminal of the secondary winging of the matrix transformer 80 is connected to the ground GND.
The sound signals from the mid microphone unit 20 and the side microphone unit 30 are subjected to impedance conversion, and then converted to the signals M+S and the signals M−S through the buffer amplifiers and the matrix transformers 70 and 80, and then output as left channel signals and right channel signals, respectively. This is described in further detail. On the side of the mid microphone unit 20, the signal M is output from the buffer amplifier mainly including the transistor 45. Among the sound signals from the side microphone unit 30, the signal −S is output from the collector of the transistor 65 as a main component of the buffer amplifier, and the signal +S is output from the emitter of the transistor 65. Thus, the positive terminal of the primary winding of the matrix transformer 70 receives the signal M, and the negative terminal thereof receives the signal −S, and thus the positive terminal of the secondary winding of the matrix transformer 70 outputs the signal M+S, as a left channel signal from the output terminal L. In addition, the positive terminal of the primary winding of the matrix transformer 80 receives the signal M, and the negative terminal thereof receives the signal +S, and thus the positive terminal of the secondary winding of the matrix transformer 80 outputs the signal M−S, as a right channel signal from the output terminal R.
In this way, the conventional MS stereo microphone, which includes a unidirectional or narrow directional microphone unit for the mid microphone unit and a bidirectional microphone unit for the side microphone unit, can output stereo sound signals separated for left and right channels through electroacoustic conversion. The disadvantage of the conventional MS stereo microphone, however, is use of the bidirectional microphone unit. To achieve bidirectionality of a capacitor microphone unit, fixed electrodes must be oppositely disposed on two sides of a diaphragm while acoustic resistances must be symmetrically disposed on the two sides, in order to adjust the value of each acoustic resistance. In addition, in the conventional MS stereo microphone, such a symmetric layout of the acoustic resistances sandwiching the diaphragm does not always provide the bidirectionality unless an optimum design is found for the bidirectionality. Furthermore, the bidirectional microphone unit is expensive compared with the unidirectional microphone unit. In addition, the conventional MS stereo microphone must have a matrix circuit for obtaining a sum signal and a difference signal of the signals M and S, so that cost inevitably increases. In the example shown in FIG. 12, the matrix transformers 70 and 80 function as matrix circuits. Instead, each matrix circuit may include an active element such as a transistor. In each case, the matrix circuit causes an increase in cost.
Japanese Unexamined Patent Application Publication No. 2006-174136 (W-A-2006-174136) describes an MS stereo microphone. This stereo microphone includes a unidirectional mid unit and a bidirectional side unit facing perpendicularly to each other. A fixed electrode of the mid unit is electrically connected to a diaphragm of the side unit, and a first fixed-electrode of the side unit is connected to the gate of a first FET, and a second fixed-electrode of the side unit is connected to the gate of a second FET. Furthermore, in the stereo microphone described in JP-A-2006-174136, a diaphragm of the mid unit and sources of the first and second FETs are connected to ground, and drains of the first and second FETs provide stereo output.
The microphone described in JP-A-2006-174136 does not require the above-described matrix circuit, but must include a bidirectional microphone unit as a side unit, causing an increase in cost.
Japanese Unexamined Patent Application Publication No. H05-219590 (JP-A-H05-219590) describes a stereo microphone, similar to a MS stereo microphone, which includes a unidirectional or bidirectional mid unit, a unidirectional left-side-unit, and a unidirectional right-side-unit. The mid unit is disposed such that the directional axis thereof aligns to the major axis of a microphone body. The left and right side units are disposed symmetrically with respect to the major axis such that the directional axes thereof are orthogonal to the major axis. A signal output from the mid unit and a signal output from the left side unit are added by an adder, and the added signal is output as a left side signal. The signal output from the mid unit and a signal output from the right side unit are added by another adder, and the added signal is output as a right side signal.
The stereo microphone described in JP-A-H05-219590 does not require a bidirectional microphone unit, but must include the adders, resulting in an increase in cost.